The trend toward miniaturization of electronic circuitry and equipment is well documented in the art. In the field of non-removable media rotating disk data storage devices, usually called "disk drives", this trend has been followed, with a definite correlation between the size of floppy disk drives and fixed disk drives. Thus, eight inch diameter floppy disk drives were followed by eight inch diameter Winchester disk drives, see for example the present assignee's Reissue U.S. Pat. No. 32,075, as an example of a higher capacity, lower cost fixed disk drive. Five and one quarter inch diameter "mini-floppy" disk drives were followed by five and one quarter inch Winchester disk drives, see for example, the assignee's U.S. Pat. No. 4,639,798 which depicts a five and one quarter inch diameter Winchester disk drive as the preferred embodiment (but teaches applicability of the principles disclosed therein to "systems employing either larger or smaller storage disks").
With the introduction of three and one half inch diameter "micro-floppy" disk drives by Sony Corporation, the well developed design trend to follow the floppy disk drive exterior form factor with a corresponding form factor for a Winchester disk drive occurred as an obvious development, see for example, the Rodime PLC U.S. Pat. No. 4,568,988, for a description of a 10 Megabyte micro-Winchester disk drive employing a stepping motor driven head positioner.
As was true about floppy disk drives before them, most early "floppy" form factor Winchester disk drives employed step motor driven head positioning actuators. Such motors intrinsically provided magnetic detents which were used to define the boundaries of each concentric data track followed by the data transducer slider which flew upon an air bearing very close to the data surface. The foregoing referenced disk drive disclosures of the prior art therefore followed a low level computer interface convention as had been first employed with floppy disk drives, where a disk drive controller of the host computer commanded track seek operations at the drive by delivering a stream of stepping pulses to the disk drive, and simultaneously by indicating to the disk drive the direction of stepping, whether toward or away from the periphery of the data storage disks. In the case of eight inch disk Winchester disk drives, the de facto interface standard quickly became the Shugart Associates' SA 1000 interface. In the case of the five and one quarter (and later the three and one half) inch diameter Winchester disk drives, the interface convention became known as the Seagate Technology ST-506 interface (later called the ST-412 interface).
One recognized drawback of open-loop stepping motor positioners has been the relative slowness of such motors and the limited precision thereof. Average track access times of e.g. 65 milliseconds has been typical of recent micro-Winchester disk drives employing stepping motor actuators; and, open-loop, step motor precision has been limited to about 600 tracks per inch.
Quasi-closed-loop optical encoder based positioning systems have also been proposed in the prior art. Those systems typically employed voice coil actuators, either linear or rotary, to which a moving scale was attached. The scale moved between a fixed light source and a fixed photodetector array in a manner that generated quadrature position signals. The optical encoder provided track position information. Centerline information was read from a single sector servo area on the servo surface which was hidden or data masked from the controller by lying between an internal index marking signal and a delayed interface index signal. This approach enabled track densities greater than were achieved with the open loop stepper motor positioner approaches while remaining a low cost alternative and without loss of compatibility with the open loop step motor based SA1000 (8 inch) and ST 412 (51/4 inch) industry standard interfaces. Examples of such approaches are to be found in the assignee's Reissue U.S. Pat. No. 32,075, U.S. Pat. No. 4,419,701 and 4,639,798.
Large system disk drives having a higher level interface, considerably greater data storage capacity and much faster average access times were also known in the prior art. Such interfaces were typically either ESDI, PC-AT or SCSI, and enabled disk drives to be connected directly to computer buses from which command and status words were received and sent, together with blocks of data to be stored or retrieved. High performance has usually required closed loop positioning systems employing voice coil actuators and at least one disk surface devoted entirely to servo information. This information is typically written on servo tracks which are followed by a read-only servo transducer. The overhead cost of the servo surface itself and of the servo writer required to write the servo surface, together with the costs of the read-only servo transducer and related servo channel electronics has rendered the servo surface approach more costly than the step motor detent, or quasi-closed-loop optical encoder based positioning schemes.
One example of a higher capacity disk drive employing a separate servo surface for use with a track following servo loop and an optical encoder with digitized outputs in quadrature for track seeking and for microstepping during track settling is set forth in the present assignee's U.S. Pat. No. 4,516,177.
One example of a PC-AT interface, fully integrated fixed disk expansion board employing a quasi-closed-loop, optical encoder positioned, three and one half inch diameter head and disk assembly and fully integrated controller electronics interfacing directly with a personal computer, such as the IBM Personal Computer (tm), is exemplified by the disclosure set forth in the assignee's subsidiary's U.S. Pat. No. 4,639,863.
Recently, the small computer system interface (SCSI) has obtained prominence within the "floppy disk sized" Winchester disk drive marketplace. One further example of a five and one quarter inch diameter, half height, 80 megabyte, SCSI interface Winchester disk drive with on-board high level command processing capability is disclosed in the assignee's U.S. Pat. No. 4,669,004. While this prior architecture worked well, it manifested a number of drawbacks. First, the embedded servo sector arrangement adversely affected average access time as the rotary actuator had to be slowed in order not to exceed processing time capacity for handling each servo sector during seeking operations. Second, the somewhat cumbersome software organization of this prior architecture, as well as the use of an eight bit microprocessor controller unduly taxed the controller and extended the response time to SCSI interface requests, thereby limiting data throughput rates.
In summary, while all of the assignee's foregoing disk drive patents described technology marking considerable advances in providing low cost, higher capacity Winchester disk drives, certain drawbacks have remained heretofore unsolved. Primarily, these problems have related to limitations of: a) areal data densities, b) track densities limited by thermal drift characteristics, c) average access times, d) data block throughput, e) power supply requirements, f) noise emissions, and g) prime costs of manufacture.